The present invention relates to a scroll type fluid machine that is one type of a positive displacement fluid machine, and in particular, to a scroll fluid machine in which a scroll wrap is formed of circular arcs and straight lines.
Conventional scroll type fluid machines comprise a fixed scroll and an orbiting scroll eccentrically combined with each other and having a wrap of the same shape and as the wrap form, an involute curve is used typically. If an involute curve is used, the wrap pitch is constant and, therefore, the volume change rate is also constant. Thus, if an attempt is made to increase the internal volume ratio (design volume ratio) that is the ratio of the outer-most circumferential sealed volume (stroke volume) to the inner-most circumferential sealed volume within a predetermined dimension, when the number of turns of the wrap is increased, the wrap pitch is reduced, resulting in the reduction of the orbiting radius and thus the stroke volume. As a result, it is not possible to increase the internal volume ratio significantly.
In addition, since the fixed and the orbiting scrolls are eccentrically combined with each other, an unused space exists in the outer circumferential portion, and an attempt to reduce the size of the fluid machine may result in various problems due to the stroke volume and the design volume ratio. In particular, in scroll type fluid machines of a shaft penetration type in which a main shaft for revolving the orbiting scroll penetrates through the center of the orbiting and the fixed scrolls, the inner-most circumferential sealed volume (the volume of a compression space at the initiation of discharge) is reduced an amount of the space through which the main shaft penetrates, and an attempt to reduce the size of the fluid machine may result in various problems due to the stroke volume and the design volume ratio.
With respect to the above problems, one of the known techniques, in which an outer curve of the scroll wrap is circular while an inner curve thereof is involute with a high order curve extending between the outer and the inner curve, is described in Japanese Patent Unexamined Publication No. 6-213176.
In the above conventional technique, it is not easy to work the wrap. In addition, this conventional technique has critical values for the external shape and the design volume ratio. If, for example, this conventional technique is applied to a scroll type fluid machine of a shaft penetration type, the wrap must first be wound from an outside of the main shaft due to the main shaft being at the center of the wrap. The volume of a minimum confined space formed by the wrap involute or other curves becomes larger as this space is located closer to the outer-most circumference, so the provision of a constant design volume ratio (the ratio of the volume of the compression space at the initiation of compression to the same volume at the initiation of discharge) requires an increase in the number of the turns of the wrap outwardly, thereby increasing the outside dimension of the scroll. The outside dimension of the compressor is further increased by a rotation preventing mechanism section formed at the outer circumference of an end plate disposed further outwardly of the end of the turns of the wrap for preventing the rotation of the orbiting scroll. Thus, this conventional technique cannot provide refrigerating and air-conditioning scroll compressors with a diameter of 160 mm or less at a required rated power in the order of 5 horsepower, domestic air-conditioning scroll compressors with a diameter of 110 mm or less at a required rated power in the order of 1,800 watt, or scroll compressors for domestic refrigerators with a diameter of 90 mm or less at a required rated power in the order of 240 watt.